Post hoc, ergo propter hoc
| Cogito ergo sum|
Logic and rhetoric
Post hoc, ergo propter hoc is a Latin phrase for "after this, therefore, because of this." The term refers to a logical fallacy that because two events occurred in succession, the former event caused the latter event.
In addressing a post hoc, ergo propter hoc argument, it is important to recognise that correlation does not equal causation.
Magical thinking is a form of post hoc, ergo propter hoc fallacy, in which superstitions are formed based on seeing patterns in a series of coincidences. For example, "these are my lucky trousers. Sometimes good things happen to me when I wear them."
- Assuming the cause
- Faulty Causal Assumption
- Post hoc
Form of the argument
- P1: X happened before Y.
- P2: (unstated) Y was caused by something (that happened before Y).
- C: Therefore, X caused Y.
Many superstitions use this. For instance, a black cat crosses my path on the way to school. I then fail a test that day. If I used this fallacy, I may conclude I failed because of the black cat, while ignoring other factors such as the amount of time I spent studying.
- The rooster crows before sunrise, therefore the crowing rooster causes the sun to rise.
- The drunk scientist conducts an experiment to see why he gets hangovers. He decides to keep a diary. Monday night, scotch and soda; Tuesday morning, hangover. Tuesday night, gin and soda; Wednesday morning, hangover. Wednesday night, vodka and soda; Thursday morning, hangover. Thursday night, rum and soda; Friday morning, hangover. On Friday night before going out for a drink, the drunk scientist has an epiphany. "Aha!" he says to himself, "I've got it! Soda causes hangovers!"
- In the immortal words of Andrew Schlafly: "In Romania, abortion was illegal under two decades of rule by the communist dictator Nicolae Ceausescu, and the country enjoyed one of the lowest breast cancer rates in the entire world during that time, far lower than comparable Western countries." (Just because the breast cancer rate went down does not mean that the illegality of abortions caused it.)
- When compared on a graph, the number of deaths by falling and drowning in a swimming pool correlates with the number of movies that Nick Cage is in. This is completely coincidental… Unless it's not.
The obvious caveat
Note that this fallacy can be partly logically valid, in that "Every time I've seen an X, there was a Y nearby" is usually good cause for further investigation. For example, "Just about everybody I know who smokes Lucky Strike cigarettes gets lung cancer" was a valid place to start an investigation into what was later realized to be the smoking-cancer connection — although the causation was partly wrong: by no means was the connection confined to one brand.
Just keep in mind the causation might be different than you might expect; for one obvious example, "fat people eat more diet food" does not mean "diet food causes fatness"[note 1]. Our Correlation does not imply causation article has more on the various possible causality directions of a real correlation.
Want to read this in another language?
- See the Wikipedia article on Post hoc ergo propter hoc.
- Do you know why we lost Texas?
- Post Hoc, Fallacy Files
- Post Hoc Ergo Propter Hoc, Bruce Thompson
- Spurious correlations, Tyler Vigen
- Although this particular point can be (and has been) argued about in the real world, depending on the diet food, for our purposes it's sufficient to note that people who are already fat might go for diet food in hopes of lowering their weight as a more plausible counter-explanation.